internal/buildcfg: enable unified IR by default

Unified IR is now at feature parity with the non-unified frontend.

Change-Id: I2a8cd41267a66f5eb1f425b89d18c6d501676e07
Reviewed-on: https://go-review.googlesource.com/c/go/+/422235
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/internal/buildcfg/exp.go

@@ -72,6 +72,7 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	baseline := goexperiment.Flags{
 		RegabiWrappers: regabiSupported,
 		RegabiArgs:     regabiSupported,
+		Unified:        true,
 	}
 
 	// Start with the statically enabled set of experiments.